Structures and fabrication techniques for solid state electrochemical devices

ABSTRACT

Porous substrates and associated structures for solid-state electrochemical devices, such as solid-oxide fuel cells (SOFCs), are low-cost, mechanically strong and highly electronically conductive. Some preferred structures have a thin layer of an electrocatalytically active material (e.g., Ni-YSZ) coating a porous high-strength alloy support (e.g., SS-430) to form a porous SOFC fuel electrode. Electrode/electrolyte structures can be formed by co-firing or constrained sintering processes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.12/027,183, entitled STRUCTURES AND FABRICATION TECHNIQUES FOR SOLIDSTATE ELECTROCHEMICAL DEVICES filed Feb. 6, 2008, which is acontinuation of U.S. application Ser. No. 11/471,774, entitledSTRUCTURES AND FABRICATION TECHNIQUES FOR SOLID STATE ELECTROCHEMICALDEVICES filed Jun. 20, 2006; which is a continuation of U.S. applicationSer. No. 11/260,009, entitled STRUCTURES AND FABRICATION TECHNIQUES FORSOLID STATE ELECTROCHEMICAL DEVICES filed Oct. 26, 2005; which is acontinuation of U.S. application Ser. No. 10/273,812, entitledSTRUCTURES AND FABRICATION TECHNIQUES FOR SOLID STATE ELECTROCHEMICALDEVICES filed Oct. 17, 2002; which is a divisional of U.S. applicationSer. No. 09/626,629, entitled STRUCTURES AND FABRICATION TECHNIQUES FORSOLID STATE ELECTROCHEMICAL DEVICES filed Jul. 27, 2000; which claimspriority of U.S. Provisional Application No. 60/146,769, entitled ALLOYSUPPORT STRUCTURE FOR CERAMIC ELECTROCHEMICAL DEVICES, filed Jul. 31,1999; the disclosures of which are herein incorporated by reference.

STATEMENT OF GOVERNMENT SUPPORT

This invention was made with government support under ContractDE-ACO3-76SF00098 awarded by the United States Department of Energy toThe Regents of the University of California for the management andoperation of the Lawrence Berkeley National Laboratory. The governmenthas certain rights in this invention.

BACKGROUND OF THE INVENTION

The present invention relates generally to the field of solid stateelectrochemical devices, and more particularly to substrate, electrodeand cell structures for solid state electrochemical devices.

Solid state electrochemical devices are often implemented as cellsincluding two porous electrodes, the anode and the cathode, and a densesolid electrolyte and/or membrane which separates the electrodes. Forthe purposes of this application, unless otherwise explicit or clearfrom the context in which it is used, the term “electrolyte” should beunderstood to include solid oxide membranes used in electrochemicaldevices, whether or not potential is applied or developed across themduring operation of the device. In many implementations, such as in fuelcells and oxygen and syn gas generators, the solid membrane is anelectrolyte composed of a material capable of conducting ionic species,such as oxygen ions, or hydrogen ions, yet has a low electronicconductivity. In other implementations, such as gas separation devices,the solid membrane is composed of a mixed ionic electronic conductingmaterial (“MIEC”). In each case, the electrolyte/membrane must be denseand pinhole free (“gas-tight”) to prevent mixing of the electrochemicalreactants. In all of these devices a lower total internal resistance ofthe cell improves performance.

The ceramic materials used in conventional solid state electrochemicaldevice implementations can be expensive to manufacture, difficult tomaintain (due to their brittleness) and have inherently high electricalresistance. The resistance may be reduced by operating the devices athigh temperatures, typically in excess of 900° C. However, such hightemperature operation has significant drawbacks with regard to thedevice maintenance and the materials available for incorporation into adevice, particularly in the oxidizing environment of an oxygenelectrode, for example.

The preparation of solid state electrochemical cells is well known. Forexample, a typical solid oxide fuel cell (SOFC) is composed of a denseelectrolyte membrane of a ceramic oxygen ion conductor, a porous anodelayer of a ceramic, a metal or, most commonly, a ceramic-metal composite(“cermet”), in contact with the electrolyte membrane on the fuel side ofthe cell, and a porous cathode layer of a mixedionically/electronically-conductive (MIEC) metal oxide on the oxidantside of the cell. Electricity is generated through the electrochemicalreaction between a fuel (typically hydrogen produced from reformedmethane) and an oxidant (typically air). This net electrochemicalreaction involves charge transfer steps that occur at the interfacebetween the ionically-conductive electrolyte membrane, theelectronically-conductive electrode and the vapor phase (fuel oroxygen). The contributions of charge transfer step, mass transfer (gasdiffusion in porous electrode), and ohmic losses due to electronic andionic current flow to the total internal resistance of a solid oxidefuel cell device can be significant. Moreover, in typical devicedesigns, a plurality of cells are stacked together and connected by oneor more interconnects. Resistive loss attributable to theseinterconnects can also be significant.

In work reported by de Souza and Visco (de Souza, S.; Visco, S. J.; DeJonghe, L. C. Reduced-temperature solid oxide fuel cell based on YSZthin-film electrolyte. Journal of the Electrochemical Society, vol.144,(no.3), Electrochem. Soc, March 1997. p.L35-7. 7), a thin film of yttriastabilized zirconia (YSZ) is deposited onto a porous cermet electrodesubstrate and the green assembly is co-fired to yield a dense YSZ filmon a porous cermet electrode. A thin cathode is then deposited onto thebilayer, fired, and the assembly is tested as an SOFC with good results.In work reported by Minh (Minh, N. Q. (Edited by: Dokiya, M.; Yamamoto,O.; Tagawa, H.; Singhal, S. C.) Development of thin-film solid oxidefuel cells for power generation applications. Proceedings of the FourthInternational Symposium on Solid Oxide Fuel Cells (SOFC-IV),(Proceedings of the Fourth International Symposium on Solid Oxide FuelCells (SOFC-IV), Proceedings of Fourth International Symposium SolidOxide Fuel Cells, Yokohama, Japan, 18-23 Jun. 1995.) Pennington, N.J.,USA: Electrochem. Soc, 1995. p.138-45), a similar thin-film SOFC isfabricated by tape calendaring techniques to yield a good performingdevice. However, these Ni-YSZ supported thin-film structures aremechanically weak, and will deteriorate if exposed to air on SOFCcool-down due to the oxidation of Ni to NiO in oxidizing environments.Also, nickel is a relatively expensive material, and to use a thickNi-YSZ substrate as a mechanical support in a solid stateelectrochemical device will impose large cost penalties.

Solid state electrochemical devices are becoming increasingly importantfor a variety of applications including energy generation, oxygenseparation, hydrogen separation, coal gasification, and selectiveoxidation of hydrocarbons. These devices are typically based onelectrochemical cells with ceramic electrodes and electrolytes and havetwo basic designs: tubular and planar. Tubular designs havetraditionally been more easily implemented than planar designs, and thushave been preferred for commercial applications. However, tubulardesigns provide less power density than planar designs due to theirinherently relatively long current path that results in substantialresistive power loss. Planar designs are theoretically more efficientthan tubular designs, but are generally recognized as having significantsafety and reliability issues due to the complexity of sealing andmanifolding a planar stack.

Thus, solid state electrochemical devices incorporating currentimplementations of these cell designs are expensive to manufacture andmay suffer from safety, reliability, and/or efficiency drawbacks. Somerecent attempts have been made to develop SOFCs capable of operatingefficiently at lower temperatures and using less expensive materials andproduction techniques. Plasma spray deposition of molten electrolytematerial on porous device substrates has been proposed, however theseplasma sprayed layers are still sufficiently thick (reportedly 30-50microns) to substantially impact electrolyte conductance and thereforedevice operating temperature.

Accordingly, a way of reducing the materials and manufacturing costs andincreasing the reliability of solid state electrochemical devices wouldbe of great benefit and, for example, might allow for thecommercialization of such devices previously too expensive, inefficientor unreliable.

SUMMARY OF THE INVENTION

In general, the present invention provides low-cost, mechanicallystrong, highly electronically conductive porous structures forsolid-state electrochemical devices, techniques for forming thesestructures, and devices incorporating the structures. In preferredembodiments, the invention provides a porous electrode designed for highstrength and high electronic conductivity (to lower resistive losses inthe device due to current collection). Conventional Ni-YSZ based SOFCsmay be greatly improved by application of the present invention by, forexample, casting a thin layer of Ni-YSZ on top of a porous high-strengthalloy support—this also substantially lowers the cost of the device byusing inexpensive alloy material for mechanical strength as opposed tonickel. Alternatively, alloys known to have good oxidation resistancecan be used to form a high-strength air electrode in a solid stateelectrochemical device. In this embodiment, an alloy such as Inconel 600is used to make a porous high-strength electrode onto which anelectrolyte membrane is co-fired.

The invention provides solid state electrochemical device substrates ofnovel composition and techniques for forming thinelectrode/membrane/electrolyte coatings on the novel or moreconventional substrates. In particular, in one embodiment the inventionprovides techniques for co-firing of a device substrate (often anelectrode) with an electrolyte or membrane layer to form densifiedelectrolyte/membrane films 1 to 50 microns thick, preferably 5 to 20microns thick. In another embodiment, densified electrolyte/membranefilms 1 to 50 microns, preferably 5 to 20 microns thick may be formed ona pre-fired substrate by a constrained sintering process. In some cases,the substrate may be a porous non-nickel cermet incorporating one ormore of the transition metals Cr, Fe, Cu, or alloys thereof.

In one aspect, the present invention provides a method of forming aceramic coating on a solid state electrochemical device substrate. Themethod involves providing a solid state electrochemical devicesubstrate, the substrate composed of a porous non-noble transitionmetal, a porous non-noble transition metal alloy, or porous cermetincorporating one or more of a non-noble non-nickel transition metal anda non-noble transition metal alloy. The substrate may optionally becoated with a material having high electrocatalytic activity for aspecific purpose, for example methane reformation, or oxygen or hydrogenion formation (e.g., Ni-YSZ). A coating of a suspension of a ceramicmaterial in a liquid medium is applied to the substrate material, andthe coated substrate is fired in an inert or reducing atmosphere.

In another aspect, the invention provides a solid state electrochemicaldevice. The device includes a sintered substrate composed of a porousnon-noble transition metal, a porous non-noble transition metal alloy,or a porous cermet incorporating one or more of a non-noble, non-nickeltransition metal and a non-noble transition metal alloy, and a sinteredcoating of a ceramic material on the substrate.

In yet another aspect, the invention provides a composition. Thecomposition is composed a substrate for a solid state electrochemicaldevice composed of a porous cermet incorporating one or more of thetransition metals Cr, Fe and Cu.

In other aspects, the invention provides devices in accordance with thepresent invention tailored to specific purposes, for example, oxygengenerators, gas separators, solid oxide fuel cells and syn gasgenerators.

These and other features and advantages of the present invention will bepresented in more detail in the following specification of the inventionand the accompanying figures which illustrate by way of example theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be readily understood by the followingdetailed description in conjunction with the accompanying drawing:

FIG. 1 depicts a planar design for a solid state electrochemical device.

FIGS. 2A-C depict a tubular design for a solid state electrochemicaldevice.

FIG. 3A depicts stages in a co-firing process in accordance with oneembodiment the present invention.

FIG. 3B is a flow chart depicting stages of a co-firing process inaccordance with one embodiment the present invention.

FIG. 4A depicts stages in a constrained sintering process in accordancewith one embodiment the present invention.

FIG. 4B is a flow chart depicting stages of a constrained sinteringprocess in accordance with one embodiment the present invention.

FIGS. 5A-E illustrate multi-layer substrate/electrode/electrolytestructures in accordance with various implementations in accordance withthe present invention.

FIGS. 6A and 6B show a scanning electron microscope (SEM) edge view ofthe Inconel and alumina substrate with the YSZ film, and an SEM topsurface of the YSZ film on the Inconel and alumina substrate,respectively, in accordance with one embodiment the present invention.

FIG. 6C illustrates the electrochemical performance an electrochemicaldevice incorporating the structure of FIGS. 6A and 6B.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to some specific embodiments of theinvention including the best modes contemplated by the inventors forcarrying out the invention. Examples of these specific embodiments areillustrated in the accompanying drawings. While the invention isdescribed in conjunction with these specific embodiments, it will beunderstood that it is not intended to limit the invention to thedescribed embodiments. On the contrary, it is intended to coveralternatives, modifications, and equivalents as may be included withinthe spirit and scope of the invention as defined by the appended claims.In the following description, numerous specific details are set forth inorder to provide a thorough understanding of the present invention. Thepresent invention may be practiced without some or all of these specificdetails. In other instances, well known process operations have not beendescribed in detail in order not to unnecessarily obscure the presentinvention.

In this specification and the appended claims, the singular forms “a,”“an,” and “the” include plural reference unless the context clearlydictates otherwise. Unless defined otherwise, all technical andscientific terms used herein have the same meaning as commonlyunderstood to one of ordinary skill in the art to which this inventionbelongs.

In general, the present invention provides low-cost, mechanicallystrong, highly electronically conductive porous substrates forsolid-state electrochemical devices. In preferred embodiments, theinvention provides a porous electrode designed for high strength andhigh electronic conductivity (to lower resistive losses in the devicedue to current collection). Conventional Ni-YSZ-based SOFCs may begreatly improved by application of the present invention by, forexample, casting a thin layer of Ni-YSZ on top of a porous high-strengthalloy support—this also substantially lowers the cost of the device byusing inexpensive alloy material for mechanical strength as opposed tonickel. Alternatively, alloys known to have good oxidation resistancecan be used to form a high-strength air electrode in a solid stateelectrochemical device. In this embodiment, an alloy such as Inconel 600is used to make a porous high-strength electrode onto which anelectrolyte membrane is co-fired.

The invention provides compositions and techniques for economicallyproducing solid state electrochemical cells operable at relatively lowtemperatures with good performance characteristics. The inventionprovides solid state electrochemical device substrates of novelcomposition and techniques for forming thinelectrode/membrane/electrolyte coatings on the novel or moreconventional substrates.

In particular, in one embodiment the invention provides techniques forco-firing of device substrate (often an electrode) with an electrolyteor membrane layer to form densified electrolyte/membrane films 1 to 50microns thick, preferably 5 to 20 microns thick. In this embodiment, thesubstrate material is “green”. In this application, the term “green”refers to materials that are unfired, or possibly pre-fired withsufficient heat to provide mechanical integrity to the material forhandling, but not enough to produce any substantial dimensional change(also referred to in the art as “bisque firing”). The substrate is thencoated with the electrolyte/membrane film and the assembly is heated toa temperature sufficient to sinter the substrate and densify thecoating.

In another embodiment, densified electrolyte/membrane films 1 to 50microns, preferably 5 to 20 microns, thick may be formed on a pre-fired(such that minimal or no shrinkage occurs during sintering of the film;also referred to in the art as “pre-sintered”) substrate by aconstrained sintering process.

In one embodiment, the invention provides a low-cost, robust, and highlyconductive substrate for solid state electrochemical devices. Forexample, a porous iron, chromium, copper or chrome steel alloy could beused as the porous support onto which a thin film of porous Ni-YSZ isdeposited. This alloy support has a much higher strength than Ni-YSZ,has a much lower cost, and has better electronic conductivity forcurrent collection in the device. Such metals and/or alloys are stablein the reducing fuel environment.

Alternatively, the SOFC or other ionic device (oxygen separation, etc.)could be built on the air electrode, such as is done in the case oftubular SOFC designs currently in production, for example, byWestinghouse. However, in the existing design the support is pre-firedporous LSM onto which the YSZ coating is applied by CVD-EVD, a veryexpensive process. Also, the LSM substrate does not have sufficientelectronic conductivity for highly efficient current collection. Thepresent invention makes use of a metal or metal alloy or metal alloycermet (where the metal or metal alloy is stable in an oxidizingenvironment) as the porous support. For example, a green substrate madewith powdered high chrome steel alloy with or without a ceramic ionic orMIEC phase, onto which a green ionic or MIEC film is deposited.According to one embodiment, this bilayer is co-fired under reducingconditions to yield an inexpensive, mechanically robust, poroussubstrate with high electronic conductivity and a dense ionic membrane.In this way, the ionic device could be operated at high current densitywith little penalty associated with ohmic drop across the air electrodesupport due to current collection.

Introduction

An overview of solid state device components and construction, and thetwo basic designs follows. This description is provided both by way ofbackground and introduction to the subject, and to provide design andfabrication details that may be adopted in compositions, devices, andmethods in accordance with the present invention.

FIG. 1 illustrates a basic planar design for a solid stateelectrochemical device, for example, a solid oxide fuel cell (SOFC). Thecell 100 includes an anode 102 (the “fuel electrode”) and a cathode 104(the “air electrode”) and a solid electrolyte 106 separating the twoelectrodes. In conventional SOFCs, the electrodes and electrolytes aretypically formed from ceramic materials, since ceramics are able towithstand the high temperatures at which the devices are operated. Forexample, SOFCs are conventionally operated at about 950° C. Thisoperating temperature is determined by a number of factors, inparticular, the temperature required for the reformation of methane toproduce hydrogen and reaction efficiency considerations. Also, typicalsolid state ionic devices such as SOFCs have a structural element ontowhich the SOFC is built. In conventional planar SOFCs the structuralelement is a thick solid electrolyte plate such as yttria stabilizedzirconia (YSZ); the porous electrodes are then screen-printed onto theelectrolyte. The porous electrodes are of low strength and are nothighly conductive. Alternatively, a thick porous electrode and a thinelectrolyte membrane can be co-fired, yielding a electrode/electrolytebilayer. As noted above, for the case where the electrode is a Ni-YSZelectrode of a few mm in thickness, the electrode strength is low andthe cost of raw materials high.

Methane (natural gas) is plentiful, inexpensive, and rich in hydrogen,the actual fuel for the cell, and as such, is the preferred fuel sourcefor a SOFC. Methane may be reformed to produce hydrogen at a temperatureof about 650-950° C. Therefore, it is desirable to operate a SOFC at atleast the lower end of this temperature range.

Another consideration governing the temperature at which a SOFC or anysolid state electrochemical device is operated is theelectrolyte/membrane conductivity. Conventional devices must be operatedat a high enough temperature to make the ceramic electrolytesufficiently ionically conductive for the energy producing reactions (inthe case of a SOFC; other reactions for gas separators or generators).The thickness of the solid electrolyte, typically hundreds of micronsthick, favors an operating temperature above 900° C. in order to achievean acceptable conductivity. Methods exist for forming thin electrolyteson ceramic substrates, such as EVD/CVD. However, EVD/CVD is a complexand expensive technique, and the ceramic-based devices to which thetechnique has been applied still require high operating temperatures tobe at all efficient. Unfortunately, most metals are not stable at thistemperature in an oxidizing environment and very quickly becomeconverted to brittle oxides. Accordingly, solid state electrochemicaldevices have conventionally been constructed of heat-tolerant ceramicmaterials, such as La_(1−x)Sr_(x)Mn_(y)O_(3−δ) (1≧X≧0.05) (0.95≦y≦1.15)(“LSM”), and yttria stabilized zirconia (e.g.,(ZrO₂)_(0.92)(Y₂O₃)_(0.08)) (“YSZ”). In an SOFC, this limitation is mostproblematic at the air electrode where oxidation can take place. Inother solid state electrochemical devices, such as oxygen generators,both electrodes may be in an oxidizing environment during operation ofthe device, and so both may face this problem.

Referring again to FIG. 1, the cell 100 is depicted in the form in whichit could be stacked with other like cells 110, as it typically would beto increase the capacity of the device. To be stacked, the cells requirebipolar interconnects 108 adjacent to each electrode that areelectrically, but not ionically, conductive. The interconnects 108 allowcurrent generated in the cells to flow between cells and be collectedfor use. These interconnects are typically formed into manifolds throughwhich fuel and air may be supplied to the respective electrodes (allowlateral movement of gas in channels; but not allow intermixing of gas(vertical movement)). Due to the highly exothermic combustion resultingfrom an uncontrolled mixture of hydrogen and oxygen, it is essentialthat the interconnect manifolds by well-sealed at all edges of theplaner cell. Moreover, due to required operating temperatures in excessof 900° C. (e.g., 950° C.) for conventional devices, the interconnect incontact with the air electrode may not be made of metal due to hightemperature corrosion.

Prior designs for solid state electrochemical planar stack devices haveused ceramic materials such as lanthanum chromite to form interconnects.However, lanthanum chromite is a very expensive material, sometimesaccounting for as much as 90% of the cost of a device. In addition, itis a relatively brittle material (relative to metal); less than idealfor an application requiring an absolute seal, and is significantly lessconductive than metal, resulting in resistive losses that reduce theoverall efficiency of the device. These problems have combined to makecurrent planar stack implementations impractical for commercialapplications.

An alternative solid state electrochemical device design generallyrecognized as having much reduced safety concerns and greaterreliability is depicted in FIGS. 2A-C. This design, commercialized byWestinghouse, for example, has a tubular shape. FIG. 2A depicts an axialcross-sectional view of a tubular SOFC 200. The inner tube is the airelectrode 202, again formed from a solid ceramic material such as LSM.The air electrode 202 is coated with a solid electrolyte 204 for most ofits circumference. The electrolyte is coated with the fuel electrode206. During operation, air is flowed through the interior of the tube,while fuel (generally methane that is reformed to hydrogen duringoperation of the cell) is provided outside the tube. In the case of thetubular SOFC, one of the major fabrication costs is associated with thedeposition of the electrolyte film by conventional chemical vapordeposition-electrochemical vapor deposition (CVD-EVD) techniques.

In order to get current produced by the fuel cell out, an electricallyconductive material in contact with the electrodes is required. Thematerial must also provide a chemical barrier to prevent intermixing ofthe hydrogen fuel outside the tube and the air inside. An interconnect208, again typically composed of lanthanum chromite, is provided on theair electrode 202 over that portion of the circumference not covered bythe electrolyte 204. The interconnect also typically has an interconnectcontact 210 attached to it. This arrangement also allows for thestacking of tubes, as illustrated in FIG. 2B, which depicts a stackedcell device 220 composed of four tubular cells 222, 224, 226, 228, inthis case oxygen generation cells, but otherwise as described above,stacked and interconnected (for example using Ag felt interconnectcontacts) between a positive current collector 229 and a negativecurrent collector 230.

FIG. 2C depicts a length-wise cross sectional view of a tubular solidstate electrochemical device, such as depicted in FIG. 2A. The device250 has a tubular shape formed by a porous air electrode (anode) 252, anelectrolyte 254, and a porous fuel electrode (cathode) 266, in the caseof an SOFC application of the device. The tube-shaped device has an openend 258 available for providing a gas reactant, such as air in the caseof an SOFC (as shown), or extracting a gas product, such as oxygen inthe case of an oxygen generator, and a closed end 260 to contain andseparate the gas inside the tube from that outside. In the case of aSOFC, the fuel gas, e.g., hydrogen or methane, is typically providedoutside the tube.

In this design, the seal preventing intermixing of reactant gasses, suchas hydrogen fuel and air in a SOFC, are much more easily implemented.Rather than requiring a seal around all the edges, as in a planerdevice, the tubular device need only be sealed at the open end 258 ofthe tube (or can even be sealless and allowed to leak). Moreover, thisend may be located out of the hot zone of the operational device. Thismakes the seal easier to maintain and thus renders the device more safeand reliable than conventional planar designs.

However, the tubular design has the significant drawback that currentcollection for the whole tube occurs at only a small area on thecircumference of the tube. Referring to FIG. 2A, all current coming fromall around the tube gets collected at the interconnect 208. Thus, themaximum current path 212 is about half the circumference of the tube,which may be centimeters as opposed to microns as in the case for theplanar implementation. The resistive loss inherent to this design inconventional implementations can be more fully appreciated withreference to FIG. 2B where a tubular device stack is shown. Each cell inthe stack contributes to a very large total internal resistance for thedevice 220. As a result, the tubular implementation has much lower powerdensity than the planar devices, making the development of high powerdensity devices using this design impractical. In addition, this designretains the drawback of being composed of materials (ceramic electrodesand interconnects) that contribute significantly to the device'sinternal resistive losses, thereby limiting power density

While the designs depicted and described in FIGS. 1 and 2A-C areintended for use as a SOFC, the same or a similar device designs mightalso be used for gas separation or generation depending on the selectionof materials used as the electrodes and separators, the environment inwhich the device is operated (gases supplied at each electrode),pressures or electrical potentials applied, and the operation of thedevice. For example, as described above, for a fuel cell, ahydrogen-based fuel (typically methane that is reformed to hydrogenduring operation of the device) is provided at the fuel electrode andair is provided at the air electrode. Oxygen ions (O²⁻) formed at theair electrode/electrolyte interface migrate through the electrolyte andreact with the hydrogen at the fuel electrode/electrolyte interface toform water, thereby releasing electrical energy that is collected by theinterconnect/current collector.

In the case of the fuel cell, the electrolyte is composed of a solelyionic conducting material, such as yttria stabilized zirconia (YSZ). Ifthe same device is operated as an electrolytic device, that is, ratherthan getting energy out of the device, energy is provided to the deviceas a potential applied across the two electrodes, ions formed from gas(e.g., oxygen ions from air) at the cathode will migrate through theelectrolyte (which is selected for its conductivity of ions of a desiredpure gas) to produce pure gas (e.g., oxygen) at the anode. If theelectrolyte is a proton conducting thin film (for example, doped BaCeO₃,doped SrCeO₃ or doped SrZrO₃) instead of an oxygen ion conductor, thedevice could be used to separate hydrogen from a feed gas containinghydrogen mixed with other impurities, for instance resulting from thesteam reformation of methane (CH₄+H₂O=3H₂+CO). Protons (hydrogen ions)formed from the H₂/CO mixture at one electrode/thin film interface couldmigrate across the thin film driven by a potential applied across theelectrodes to produce high purity hydrogen at the other electrode. Thusthe device may operate as a gas generator/purifier.

Such a device could also function as a electrochemical syn gasgenerator. Syn gas (H₂+CO) is a valuable product used for synthesis ofhigher value organics. It is typically produced by the partial oxidationof methane with pure oxygen. Since the pure oxygen must be separatedfrom air in a separate process, syn gas production is relativelyexpensive. In this case, the feed to the fuel electrode is methane, andair is supplied to cathode, as with the fuel cell. However, the deviceis run at a current density where methane is only partially oxidized toH₂ and CO, as opposed to deep oxidation in typical fuel cell operationto produce H₂O and CO₂.

If the solely ionic conducting electrolyte is replaced with a mixedionic electronic conducting (MIEC) membrane, such as LSM, and instead ofapplying a potential across the electrodes, air at high pressure isprovided on one side of the membrane, oxygen ions formed from the air atthe membrane will migrate through the membrane to produce pure oxygen atthe other side of the membrane. Thus the device may operate as an oxygengas separator.

Fabrication Techniques, Compositions and Substrates of the Invention

FIG. 3A depict stages in a co-firing process in accordance with oneembodiment the present invention. An unfired (“green”) solid stateelectrochemical device substrate material 302 is formed and coated witha thin layer of electrolyte/membrane material 304. The substratematerial 302 may be a cermet, for example, composed of 50 vol % Al₂O₃(e.g., AKP-30) and 50 vol % Inconel 600 (available from Powder AlloyCorp) with a small amount of binder (e.g., XUS 40303 from Dow ChemicalCompany). The cermet components may be mixed in water and dried, and theresulting powder ground and sieved, for example to less than about 100μm. The powder may be pressed (e.g., at about 5000 lbs.) into a greensubstrate layer, for example, in the form of a disk.

Of course, other solid state electrochemical device substrates may alsobe used. Suitable substrate materials in accordance with the presentinvention include other cermets, metals and alloys. Suitable ceramiccomponents for cermets include La_(1−x)Sr_(x)Mn_(y)O_(3−δ) (1≧X≧0.05)(0.95≦y≦1.15) (“LSM”), La_(1−x)Sr_(x)CoO_(3−δ) (1≧X≧0.10) (“LSC”),SrCo_(1−x)Fe_(x)O_(3−δ) (0.30≧X≧0.20),La_(0.6)Sr_(0.4)Co_(0.6)Fe_(0.4)O_(3−δ), Sr_(0.7)Ce_(0.3)MnO_(3−δ),LaNi_(0.6)Fe_(0.4)O₃, Sm_(0.5)Sr_(0.5)CoO₃, yttria stabilized zirconia(YSZ), scandia stabilized zirconia (SSZ), (CeO₂)_(0.8)(Gd₂O₃)_(0.2)(CGO), La_(0.8)Sr_(0.2)Ga_(0.85)Mg_(0.15)O_(2.825) (LSGM20-15),(Bi₂O₃)_(0.75)(Y₂O₃)_(0.25) and alumina. Preferred LSM materials includeLa_(0.8)Sr_(0.2)MnO₃, La_(0.65)Sr_(0.30)MnO_(3−δ),La_(0.45)Sr_(0.55)MnO_(3−δ). Suitable metal components for the cermetsare transition metals Cr, Fe, Cu and/or alloys such as low-chromiumferritic steels, such as type 405 and 409 (11-15% Cr),intermediate-chromium ferritic steels, such as type 430 and 434, (16-18%Cr), high-chromium ferritic steels, such as type 442, 446 and E-Brite(19-30% Cr), chrome-based alloys such as Cr5Fe1Y and chrome-containingnickel-based Inconel alloys including Inconel 600 (Ni 76%, Cr 15.5%, Fe8%, Cu 0.2%, Si 0.2%, Mn 0.5%, and C 0.08%). The substrate material mayalso be a porous metal such as transition metals chromium, copper, ironand nickel, or a porous alloy such as low-chromium ferritic steels, suchas type 405 and 409 (11-15% Cr), intermediate-chromium ferritic steels,such as type 430 and 434, (16-18% Cr), high-chromium ferritic steels,such as type 442, 446 and E-Brite (19-30% Cr), chrome-based alloys suchas Cr5Fe1Y and chrome-containing nickel-based Inconel alloys includingInconel 600 (Ni 76%, Cr 15.5%, Fe 8%, Cu 0.2%, Si 0.2%, Mn 0.5%, and C0.08%).

In some embodiments of the present invention, the substrate may be aporous non-nickel cermet incorporating one or more of the transitionmetals Cr, Fe, Cu, or alloys thereof. These metals are particularlywell-suited for use in the high temperature reducing or oxidizingenvironments of some components of solid state electrochemical devices,particularly oxidizing electrodes and interconnects, since under suchconditions they form a thin oxide surface layer having a growth rateconstant of no more than about 1×10⁻¹² cm²/sec that protects them fromfurther oxidation while they retain their beneficial metal properties.Porous substrates made from these materials preferably have a fracturestrength in excess of 5 Mpa (megapascals), more preferably 40 MPa, andstill more preferably 100 MPa. Examples of these materials include YSZ—Cr5Fe1Y,CGO—Cr5Fe1Y, YSZ—SS409, 410 or 430, and CGO—SS409, 410 or 430.

The electrolyte membrane material 304 may be a thin layer of a metaloxide (ceramic) powder, such as yttria stabilized zirconia (YSZ) (e.g.,(ZrO₂)_(0.92)(Y₂O₃)_(0.08) or (ZrO₂)_(0.90)(Y₂O₃)_(0.10)) available forexample, from Tosoh Corp. Other possible electrolyte materials include(ZrO₂)_(0.9)(Sc₂O₃)_(0.1) scandia stabilized zirconia (SSZ),(CeO₂)_(0.8)(Gd₂O₃)_(0.2) (CGO),La_(0.8)Sr_(0.2)Ga_(0.85)Mg_(0.15)O_(2.825) (LSGM20-15) and(Bi₂O₃)_(0.75)(Y₂O₃)_(0.25). Alternatively, the membrane material may bea mixed ionic electronic conductor, for example SrCo_(1−x)Fe_(x)O_(3−δ)(0.30 ≧X≧0.20), La_(0.6)Sr_(0.4)Co_(0.6)Fe_(0.4)O_(3−δ),La_(0.8)Sr_(0.2)MnO₃, La_(0.65)Sr_(0.30)MnO₃, La_(0.45)Sr_(0.55)MnO₃,Sr_(0.7)Ce_(0.3)MnO_(3−δ), LaNi_(0.6)Fe_(0.4)O₃, Sm_(0.5)Sr_(0.5)CoO₃and La_(1−x)Sr_(x)CoO_(3−δ). Such structures may find use in oxygenseparation devices, for example, as described above.

The electrolyte/membrane material 304 is generally prepared as asuspension of the powder material in a liquid media, such as water,isopropanol, and other suitable organic solvents may be applied to asurface of the pressed substrate layer by a variety of methods, forexample by one of aerosol spray, dip coating, electrophoreticdeposition, vacuum infiltration, and tape casting.

At this stage, both materials are green; that is, neither material hasyet been fired to a temperature sufficiently high to sinter thematerials. As is known in the art, sintering refers to a process offorming a coherent mass, for example from a metallic powder, by heatingwithout melting. The resulting coated substrate assembly may be pressedagain (e.g., to about 6000 psi.) to increase the green density of theYSZ electrolyte film and enhance electrode electrolyte adhesion. Then,the assembly may be fired in an inert or reducing atmosphere at atemperature sufficient to sinter the substrate and densify theelectrolyte, for example at about 1200-1500° C. In one example theassembly may be placed film side down in a molybdenum furnace under 2psi flowing He. The furnace may be heated initially to 450° C., and thenat a rate of 5° C./min to 1350° C., held for 4 hr, and then cooled at arate of 5° C./min.

As shown in FIG. 3A, the fired electrode/electrolyte bilayer willshrink, for example, on the order of about 10% as the materials sinterand the electrolyte/membrane densifies. The fired electrolyte 314 mustbe sufficiently densified to provide a gas-tight barrier between thegases at each electrode. The fired electrode 312 is preferably at least90% densified (about 10% porosity), and may be as much as about 95%densified, or even about 98% densified. After the co-firing, thesubstrate remains porous, in one embodiment to less than about 80% dense(preferably about 60 to 70% dense (about 30 to 40% porosity), in orderto allow gases to diffuse through the electrode (where the substrate isan electrode) or to it (where the substrate is a support for anelectrode, as discussed further below).

The thickness of the densified electrolyte films prepared in accordancewith preferred embodiments of the present invention may be from about 1to 50 microns; more preferably from about 3 to 30 microns; even morepreferably about 5 to 20 microns. The fabrication of such thin,substantially gas-tight solid oxide films in an economical manner is animportant aspect of the present invention with distinct advantages overthe thicker, more expensive and/or more difficult to fabricateelectrolyte layers of conventional solid state electrochemical devicefabrication.

In one preferred embodiment the substrate is a porous metal, such as Nior Cr, or even more preferably, a metal alloy such as FeCr and isselected so that its coefficient of thermal expansion (CTE) is close(e.g., within about 20%; preferably within about 10%; more preferablywithin about 2%) to that of the metal oxide electrolyte film (or othermaterial layer) to be supported. This type of CTE matching of thesubstrate and thin film layer may be easily determined from literaturesources or with minimal experimentation by those skilled in the art.

In such cases where the substrate is a metal or alloy, it is importantthat the assembly be fired at a temperature sufficient to result insintering of the green electrolyte film without reaching the meltingpoint of the substrate. Since the atmosphere during sintering isselected to be inert or reducing, the metal or alloy substrate does notsubstantially oxidize and therefore retains it beneficial metalproperties. In many cases, solid state electrochemical devicesincorporating structures in accordance with the present invention willbe capable of efficient operation at temperatures below those at whichthe metals or alloys would oxidize too rapidly in an oxidizingenvironment (e.g., about 650-800° C.). Depending on the composition ofthe various components within the parameters defined herein, devices inaccordance with the present invention may be suitable for operationthrough a range of temperatures from about 400 to 1000° C. In oneembodiment, SOFC devices in accordance with the present invention areoperated at about 650° C. so that methane may be reformed for hydrogenfuel during operation of the device. In other embodiments, devices maybe effectively operated at temperatures as low as about 400° C.

Moreover, where the porous substrate is used in a reducing or fuelenvironment, the metal or metal alloy is inherently stable and thus maybe formed from a wider array of metals. For example, a porous iron orInconel support may be used as the robust structural element for thefuel electrode in a SOFC, on top of which a thin layer of a moreelectrochemically active material, such as Ni-YSZ, may be used forcharge/transfer reactions.

The ability to operate solid state electrochemical devices attemperatures below 800° C. provided by the present invention is animportant advantage over conventional technology. At such operatingtemperatures, metals may be used to fabricate components of devices,including substrates and electrodes, as described above, and alsoimportantly interconnects. The ability to use metals rather thanceramics such as lanthanum chromite for interconnects makes sealing ofelectrochemical devices much easier. Metals are much easier to machinethan ceramics, and may be worked with in conventional ways to form goodseals and electrical contacts, such as by welding, brazing, etc. Thus,planar electrochemical device designs, with all their low internalresistance characteristics, may be made economically feasible andreliable.

Moreover, the material cost of metal is much lower than the rare earthelements that go into the ceramic and cermet materials they replace.Metals also have higher thermal conductivity than ceramics or cermetswhich results in results in lower heat-related stresses and reduces theneed for cooling with excess gas. And metals have higher electricalconductivity, resulting in decrease internal resistance and thereforeimproved performance of solid state electrochemical devices. Metals areless brittle and therefore less susceptible to catastrophic failure thanceramics allowing the fabrication of larger cells and devices.

As discussed further below, an intermediate layer, for example, anporous electrode layer having a composition such as described above forthe support material may be applied between the support material and theelectrolyte layer. Further, as described further below, multiple porouslayers may be applied to the opposing side of the metal oxideelectrolyte layer, and on the opposing side of the porous substrate.

FIG. 3B is a flow chart depicting stages (350) of a co-firing process inaccordance with one embodiment the present invention. A green substratematerial is formed or provided according to the parameters describedabove (352). A green metal oxide material, such as a precursor forforming a solid oxide electrolyte is applied to a surface of thesubstrate, for instance as a suspension as described above (354). Thegreen substrate and metal oxide coating are then co-fired in an inert orreducing environment at a temperature sufficient to sinter the substratematerial into a porous substrate layer and the sinter and densify themetal oxide layer into a gas-tight electrolyte membrane (356). Co-firingprovides the additional benefit of improved interfacial properties atthe newly-formed substrate/membrane interface since the substrate andcoating materials sinter together at the interface.

According to another embodiment of the present invention, a green metaloxide coating may be sintered on a pre-fired porous substrate in aprocess referred to as constrained sintering. FIG. 4A depicts stages ina constrained sintering process in accordance with one embodiment thepresent invention, and FIG. 4B is a flow chart depicting stages (450) ofa constrained sintering process in accordance with one embodiment thepresent invention. The materials and techniques used to form thesubstrate and coating components may be otherwise the same as describedabove with reference to the co-firing process. However, in thisembodiment, the substrate 402 is pre-fired (pre-sintered), for exampleat about 1200-1500° C. (e.g., 1350° C.) in a molybdenum or graphitefurnace (452). A green metal oxide material 404 is then applied to thepre-fired substrate (454), for example in a suspension as describedabove, and the metal oxide coating is sintered in a reducing atmosphereon the substrate, for example at about 1200-1500° C. (e.g., 1350° C.) ina molybdenum or graphite furnace.

In the case of constrained sintering, the pre-fired porous substrate 402is invariant, so it is only the coating material layer that shrinksduring sintering. The final properties of the substrate 402 and thesintered electrolyte layer 414 should be the same as described abovewith reference to the co-firing process. That is, the substrate shouldbe porous with about 20% or greater porosity in one embodiment, and theelectrolyte membrane should be densified so as to provide a gas-tightmembrane, generally at least about 90% densified.

Structures

As noted above, structures and devices in accordance with the presentinvention may also include a plurality of layers on either side of thesubstrate/electrolyte composite described above. Moreover, it ispossible to add a separate thin electrode layer on the substrate layerintermediate between the substrate and the electrolyte. For example, insome instances it may facilitate processing, increase strength and/ordecrease device cost to produce a porous substrate from one materialthat may not have high electrocatalytic activity selected for aparticular purpose, and then form a thin electrode on that substratematerial according to well known processing techniques, before adding anelectrolyte layer according to the procedures described herein. Forexample, on the substrate side an additional layer may be spray-coatedor vacuum infiltrated onto the substrate, then the electrolyte layer maybe added and the whole assembly co-fired. A second electrode could addedas a green layer on the green electrolyte layer and then co-fired withthe other two layers, or it could be added after the co-firing of theother two layers and bonded in a second firing step.

FIGS. 5A-E illustrate multi-layer structures in accordance with variousimplementations in accordance with the present invention. FIG. 5A showsa densified electrolyte layer 502 on a porous substrate 504. In thisembodiment, the substrate may act as an electrode.

FIG. 5B shows a porous electrode layer 512 on a densified electrolytelayer 514 on a porous substrate 516. Where the substrate acts as asecond electrode, this structure forms an electrochemical cell.

FIG. 5C shows a densified electrolyte layer 522 on a porous electrodelayer 524 on a porous substrate 526. This implementation may be usedwhere, for example, the substrate 526 is composed of an inexpensive highstrength material, such as a metal, and the electrode layer 524 iscomposed of a material having high electrocatalytic activity for aspecific purpose, for example methane reformation, or oxygen or hydrogenion formation. As noted above, conventional Ni-YSZ-based SOFCs may begreatly improved in accordance with this aspect of the present inventionby, for example, casting a thin layer of Ni-YSZ on top of a poroushigh-strength alloy support—this also substantially lowers the cost ofthe device by using inexpensive alloy material for mechanical strengthas opposed to nickel.

FIG. 5D shows a porous second electrode layer 532 on a densifiedelectrolyte layer 534 on a porous first electrode layer (having highelectrocatalytic activity, e.g., Ni-YSZ) 536 on a porous substrate(which may not have high electrolcatalytic activity, but may have otherbenefits, such as mechanical strength and/or low cost, e.g.,intermediate-chromium ferritic steels, such as type 430 and 434) 538.This implementation completes a cell for an SOFC implementation such asthat described with reference to FIG. 5C.

FIG. 5E shows a porous second electrode layer 542 on a densifiedelectrolyte layer 544 on a porous first electrode layer 546 on a poroussubstrate 548, such as that described with reference to FIGS. 5C and 5D.The porous substrate is bonded to an interconnect 550, in a preferredembodiment of the invention a metal interconnect.

The techniques described herein, and the structures they produce may beused in the fabrication of a variety of electrochemical devices, asdescribed above, to reduce cost, improve performance and reliability,and reduce operating temperature for an efficient device. It should beunderstood that the fabrication techniques and structures describedherein may be implemented in either planar or tubular solid stateelectrochemical device designs.

EXAMPLES

The following examples describe and illustrate aspects and features ofspecific implementations in accordance with the present invention. Itshould be understood the following is representative only, and that theinvention is not limited by the detail set forth in these examples.

Example 1 Co-Firing of YSZ Film on Inconel-Alumina Substrate

50 vol % Al₂O₃ (AKP-30) and 50 vol % Inconel 600 (Powder Alloy Corp.)with a small amount of binder (XUS 40303 from Dow Chemical Company) weremixed in water and dried. The resulting powder was ground in a mortarand pestle and sieved to <100 μm. 5 g of the powder was pressed to 5000psi in an 1.5 in steel die. A thin layer of YSZ powder (Tosoh Corp.) wasapplied to one surface of the pressed disk. The coated disk was againpressed to 6000 psi to increase the green density of the YSZ film andenhance adhesion. The disk was placed film side down in a molybdenumfurnace under 2 psi flowing He. The furnace was manually heated to 450°C., and then at a rate of 5° C./min to 1350° C., held for 4 hr, and thencooled at a rate of 5° C./min. The resulting bilayer shrank 10%. Thefilm side was shiny and clear, the uncoated side was slightly pinkish(chromia scale). FIGS. 6A and 6B show a scanning electron microscope(SEM) edge view of the Inconel and alumina substrate with the YSZ film,and an SEM top surface of the YSZ film on the Inconel and aluminasubstrate respectively. FIG. 6C illustrates the electrochemicalperformance an electrochemical device incorporating this structure at750° C.

CONCLUSION

Although the foregoing invention has been described in some detail forpurposes of clarity of understanding, those skilled in the art willappreciate that various adaptations and modifications of thejust-described preferred embodiments can be configured without departingfrom the scope and spirit of the invention. Moreover, the describedprocessing distribution and classification engine features of thepresent invention may be implemented together or independently.Therefore, the described embodiments should be taken as illustrative andnot restrictive, and the invention should not be limited to the detailsgiven herein but should be defined by the following claims and theirfull scope of equivalents.

1. A method of separating oxygen from air, comprising: providing anelectrochemical device having a membrane, the membrane comprising asintered substrate consisting essentially of a material selected fromthe group consisting of a porous non-noble transition metal, a porousnon-noble transition metal alloy, and a porous cermet incorporating oneor more of a non-noble, non-nickel transition metal and a non-nobletransition metal alloy; and a sintered coating of a ceramic material onthe substrate wherein said coating is a mixed ionic electronicconductor; providing air on one side of the membrane, whereby oxygenions migrate through the membrane to produce pure oxygen at the otherside of the membrane.
 2. The method of claim 1, wherein said mixed ionicelectronic conductor is selected from the group consisting ofSrCo_(1−x)Fe_(x)O_(3−δ) (0.30≧X≧0.20),La_(0.6)Sr_(0.4)Co_(0.6)Fe_(0.4)O_(3−δ), La_(0.8)Sr_(0.2)MnO₃,La_(0.65)Sr_(0.30)MnO₃, La_(0.45)Sr_(0.55)MnO₃,Sr_(0.7)Ce_(0.3)MnO_(3−δ), LaNi_(0.6)Fe_(0.4)O₃, Sm_(0.5)Sr_(0.5)CoO₃and La_(1−x)Sr_(x)CoO_(3−δ).
 3. The method of claim 2, wherein said MIECis La_(0.6)Sr_(0.4)Co_(0.6)Fe_(0.4)O_(3−δ).
 4. The method of claim 1,wherein the membrane film is 1 to 50 microns thick.
 5. The method ofclaim 4, wherein the membrane film is 5 to 20 microns thick.
 6. Themethod of claim 1, wherein the device is tubular.
 7. The method of claim1, wherein the device is planar.
 8. The method of claim 1, wherein inthe providing of the electrochemical device the sintered coating is madeby co-firing of the substrate and coating.
 9. The method of claim 1,wherein in the providing of the electrochemical device the sinteredcoating is made by constrained sintering of the substrate and coating.10. The method of claim 1, wherein the electrochemical device comprises:the sintered substrate consisting essentially of a material selectedfrom the group consisting of a porous non-noble transition metal, aporous non-noble transition metal alloy, and a porous cermetincorporating one or more of a non-noble, non-nickel transition metaland a non-noble transition metal alloy; a first porous electrodematerial having high electrocatalytic activity on the substrate; thesintered coating of mixed ionic electronic conductor ceramic material onthe electrode wherein said coating is dense; and a second porouselectrode on the coating.